You can check whether the port is open on a Server by using tcping.exe which you can download online. Tcping can also be similarly used to check other ports on a Server.
Open Powershell or Command prompt in Admin mode and then type the following command:
tcping smtp.some.domain 25
Port 25 is usually the default port for SMTP communication between mail servers.
You can also telnet to test out this port. Telnet should be enabled on your machine.
telnet smtp.some.domain 25
You can send a test email using Powershell using the below command:
Send-MailMessage -From 'Test User1 <email@example.com>' -To 'Test User <firstname.lastname@example.org>' -Subject 'Test mail' -SmtpServer 'smtp.some.domain'